Stereolithography is a method of rapid prototyping engineering (RPE) with which any complex plastic models can be produced rapidly with the help of 3D CAD model data. In this method, a thin layer of a liquid reaction resin is exposed to a computer-controlled laser beam and cured in some areas. The (partially) cured structure on a platform is then lowered into the reaction resin which is in a container, and after coating (with fresh reaction resin), this new layer is again exposed to and cured with the laser in some areas. This yields the three-dimensional model layer by layer.
The photopolymers, i.e., the reaction resin mixtures used in this process, must meet a number of requirements, some of which are contradictory:
High reactivity, permitting solidification of the selected layer thickness at a minimal exposure energy, i.e., a high laser scan rate, which is thus crucial for short and therefore economical production times. PA1 High green strength, i.e., adequate mechanical strength of the partially cured part formed in the photopolymer bath. This is necessary for a dimensionally accurate manufacturing process and nondestructive handling until postcuring of the part. PA1 Minimal shrinkage during curing, which means minimal curl during the manufacturing process and thus optimal dimensional accuracy. Linear shrinkage in the horizontal direction is critical for low curl, because due to the layer-by-layer manufacturing process, a partially cured layer is always linked to a newly formed layer approx. 100 .mu.m thick. Due to the reaction shrinkage of the newly generated layer, a tension is exerted on the partially hardened layers underneath, leading to distortion and the familiar curl phenomena. PA1 Good mechanical properties of the cured parts, thus permitting function tests on the parts and the subsequent processes conventionally performed in RPE technology. PA1 Low viscosity of &lt;1500 mPa.cndot.s at 25.degree. C., which is a prerequisite for optimal recoating, i.e., for short waiting times until planarization of the photopolymer layers after the layer production process. PA1 a cationically curable monomer and/or oligomer (component A), PA1 an initiator with the following structure: ##STR2## where the following applies: R.sup.1 and R.sup.2 denote, independently of one another, an alkyl with one to nine carbons (linear or branched) or a cycloalkyl with four to nine carbons or together they may form a divalent aliphatic group with four to seven carbons, i.e., together with the S atom they may form a heterocyclic ring, PA1 R.sup.3 is H or an alkyl with one to nine carbons (linear or branched), PA1 R.sup.4, R.sup.5, R.sup.6 and R.sup.7, independently of each other, denote H an alkyl or alkoxy, each with one to nine carbons (linear or branched), where at least one of the R.sup.4 to R.sup.7 groups is an alkyl or alkoxy, PA1 X.sup.- is a non-nucleophilic anion such as hexafluoroantimonate (SbF.sub.6.sup.-), hexafluoroarsenate (AsF.sub.6.sup.-) and hexafluorophosphate (PF.sub.6.sup.-), tetraphenyl borate (B(C.sub.6 H.sub.5).sub.4.sup.-), tetra(perfluorophenyl) borate (B(C.sub.6 F.sub.5).sub.4.sup.-) or trifluoromethanesulfonate (CF.sub.3 --SO.sub.3.sup.31 ), PA1 plus optionally filler, pigment and/or additive. PA1 simultaneously with the UV exposure or in a subsequent process. The curing temperature is generally between 80.degree. and 200.degree. C., preferably approximately 80.degree. to 150.degree. C.
On the basis of this complex spectrum of requirements for stereolithography photopolymers, most known resin mixtures are composed of several components. The most reactive photopolymers for stereolithography at the present time are mixtures of acrylate functional compounds which react as part of a free-radical polymerization. However, a disadvantage of this chemical basis is a relatively high reaction volume shrinkage, which leads to deformation and deviations in dimensions. Therefore, with acrylates it is impossible to achieve a balanced level of mechanical properties of engineering thermoplastics from the standpoint of high strength and high elongation at tear; instead, relatively brittle molded materials are generally obtained. Furthermore, low molecular-weight acrylates such as those used to reduce viscosity are objectionable from the standpoint of occupational hygiene.
Epoxy resins are a definite improvement with regard to mechanical properties and shrinkage. They can be cured by UV-initiated cationic polymerization. The curing rate in cationic polymerization of epoxy resins is lower in comparison with that of free-radical polymerization of acrylates, and curing requires a higher UV dose, so mixed systems of epoxy resins and a class of quick-curing compounds are generally used. For example, International Patent Application No. 92/20014 discloses cationically curable mixtures of vinyl ethers and epoxy resins, and European Patent Application No. 605,361 A2 discloses mixtures of acrylates and epoxy resins for stereolithography.
For compensation of the reduced reaction rate of cationically curable photopolymers, it is desirable to use the most powerful possible UV lasers. These are available, for example, with argon ion lasers (with wavelengths of 351 and 364 nm) and with frequency-tripled Nd:YAG lasers (with a wavelength of 351 nm), but the triarylsulfonium salts (with non-nucleophilic anions such as hexafluoroantimonate and hexafluorophosphate) that are usually used for initiation of cationic polymerization absorb to an adequate extent only in the wavelength range up to approximately 340 nm. Light of a longer wavelength, however, is not absorbed effectively and thus cannot be used in practice for initiating a curing reaction.
In stereolithography it is often desirable to intentionally leave unexposed areas in the plastic model through special exposure strategies or to keep the conversion of functional groups in the exposed areas during laser exposure relatively low. This may be expedient to minimize shrinkage effects or to save on exposure time and in this way permit more economical production of the models, i.e., by shortening production time. In these cases, however, these areas must be cured in a subsequent process. However, subsequent exposure of the finished model with UV light is not effective. For rapid laser curing of the thin layers, high concentrations of UV-absorbing initiators must be used, which also leads to absorption of light in the layers near the surface in the finished part; therefore, deeper areas are not accessible to UV postcuring. A lower initiator content or a lower optical density would also result in curing in deeper areas than desired and thus would cause inferior dimensional stability of the parts. However, thermal postcuring of unexposed areas is impossible, because triarylsulfonium salts have a high thermal stability. An increase in temperature simultaneously with or following UV exposure does accelerate the curing reaction and cause the reaction conversion to be complete, but these effects are observed only in those areas where cations were formed from the initiator in the preceding UV process. All other areas remain liquid with no change.
There is also a demand for cationically polymerizable reaction resins that are stable in storage in application areas where thicker layers must be produced, even when light-scattering or light-absorbing additives such as fillers, pigments and coloring agents are present in the mixture. Then the light is absorbed or scattered to a very great extent in the layer areas near the surface, so that the light transmitted into deeper layers is not sufficient to induce adequate curing.
Furthermore, UV curing is impossible when there are areas due to the process that are not accessible to direct exposure. In gluing together non-transparent joining parts as well as electronic components and assemblies, the adhesive is applied first and then the component is placed on it. By exposing to UV light, only the edge areas where adhesive pours out can be cured, but curing underneath the component must be induced by an additional process, e.g., thermally. The situation is comparable when components and assemblies are provided with a protective coating to protect them from ambient influences. Because of capillary forces, the lacquer migrates below the components, where again it cannot be cured by laser exposure.
It is known from U.S. Pat. No. 4,417,061 that phenacylsulfonium salts can induce cationic polymerization under the effect of UV exposure. However, it is also known that this is effectively possible only up to a wavelength of 330 nm (see Photopolymere [Photopolymers], VEB Deutscher Verlag fur Grundstoffindustrie, Leipzig 1988, page 105); sensitizers must be used in the longer wavelength range. For example, it is known from U.S. Pat. No. 4,442,197 that dialkylphenacylsulfonium salts can be sensitized with aromatic hydrocarbons in particular. In addition, it is known that dialkylphenacyl compounds--together with reducing agents--can induce cationic polymerization at high temperatures. Dialkylphenacyl compounds of this type, however, have a low solubility in the cationically polymerizable reaction resin mixtures used industrially and crystallize out again over a period of time. These compounds thus cannot be used in practice if the mixtures must remain stable over a long period of time.
It is also known that electron shifting substituents in para position accelerate photolysis of dialkylphenacylsulfonium salts by stabilizing cationic intermediates (Macromolecules, vol. 16 (1983), pages 864-870). This corresponds to findings according to which the thermal reactivity of 4-methoxybenzylthiolanium hexafluoroantimonate is much higher than that of the unsubstituted compound (Makromolekulare Chemie [Macromolecular Chemistry], vol. 192 (1991), pages 655-662); specifically, effective cationic polymerization of styrene is observed starting at temperatures as low as approximately 40.degree. C. With such mixtures, however, the viscosity cannot be expected to remain constant for weeks at room temperature.